as in treacherous
formal
causing harm in a way that is gradual or not easily noticed
Most people with this insidious disease have no idea that they are infected. an insidious enemy
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Recent Examples of insidious
While the two play cat and mouse, an insidious supervillain in Kenneth Branagh looms overhead, threatening to take Jordan down.
—
Matt Donnelly, Variety, 16 Apr. 2026
Known for millennia and first described clinically in 1789, polio was insidious, often striking children.
—Encyclopedia Britannica, 9 Apr. 2026
After all, offenses committed by individuals in positions of public trust are especially insidious.
—Craig MacLellan, Boston Herald, 3 Apr. 2026
The trailer released this week (below) shows the gang during happier times in 1985 — before the latter seasons would introduce villain Vecna (played by Jamie Campbell Bower in the original series) and an even more insidious era of the Upside Down.
—Jackie Strause, HollywoodReporter, 27 Mar. 2026
See All Example Sentences for insidious
Matt Donnelly, Variety, 16 Apr. 2026
Known for millennia and first described clinically in 1789, polio was insidious, often striking children.
—Encyclopedia Britannica, 9 Apr. 2026
After all, offenses committed by individuals in positions of public trust are especially insidious.
—Craig MacLellan, Boston Herald, 3 Apr. 2026
The trailer released this week (below) shows the gang during happier times in 1985 — before the latter seasons would introduce villain Vecna (played by Jamie Campbell Bower in the original series) and an even more insidious era of the Upside Down.
—Jackie Strause, HollywoodReporter, 27 Mar. 2026
See All Example Sentences for insidious
Recent Examples of Synonyms for insidious
Adjective
-
Ships have increasingly traveled through the Panama Canal as shipments are rerouted and buyers purchase from other countries to avoid commerce through now-treacherous Middle Eastern waterway.—
Alma Solis,
Fortune,
24 Apr. 2026
-
Ben hunts, and Sasha tries to survive, not just him but various treacherous and dangerous elements that make that mountain climb at the start look like kids play.—
Pete Hammond,
Deadline,
23 Apr. 2026
Adjective
-
Throughout her trip to the United States, the Queen has reliably chosen British designers in a subtle showing of sartorial support of her homeland.—
Elise Taylor,
Vanity Fair,
29 Apr. 2026
-
Such subtle tweaks in rhythm and resolution are more subtle, and more rewarding, than a straightforward joke-and-punchline setup.—
Alison Herman,
Variety,
29 Apr. 2026
Adjective
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But that confidence could harbor false hope, says Kelly Richardson, principal of Richardson Ober LLP, a California law firm known for offering community association advice, and a monthly contributing writer to the Southern California News Group.—
Amancai Biraben,
Oc Register,
24 Apr. 2026
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The indictment also charges Meade and West each with one count of conspiracy to obstruct and defeat the Internal Revenue Service, five counts of attempts to interfere with administration of internal revenue laws and seven counts of aiding or assisting the filing of false documents, officials said.—
Justin Muszynski,
Hartford Courant,
24 Apr. 2026
Adjective
-
Guest reveals concerns about deceptive display.—
,
FOXNews.com,
28 Apr. 2026
-
Chicago arts venues are united in their disdain for the deceptive practice.—
Noel Brennan,
CBS News,
27 Apr. 2026
Adjective
-
The government took a genuine invitation to collaborate as a perfidious power grab.—
Gideon Lewis-Kraus,
New Yorker,
14 Mar. 2026
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For those trying to read Iran’s future, the lesson of 1953 is not just that foreign powers can be perfidious.—
Bobby Ghosh,
Time,
5 Mar. 2026
Adjective
-
Moreover, most of the assertions made by those seeking to undermine the dark matter hypothesis are now demonstrably incorrect, ruled out by the existing data.—
Big Think,
Big Think,
29 Apr. 2026
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Administrative errors or outdated income information can also lead to incorrect withholding amounts, which may temporarily increase what's taken until corrected.—
Angelica Leicht,
CBS News,
28 Apr. 2026
Adjective
-
Perhaps the owner’s unexpected absence was an indication that the rumors about my imminent demise were wrong and that things were not so certain.—
Saïd Sayrafiezadeh,
New Yorker,
26 Apr. 2026
-
After a week where everything that could have gone wrong did and the noise surrounding the club threatened to swallow the team whole, the Red Sox finally broke their four-game slide in grand fashion and got to enjoy a nice, drama-free win.—
Mac Cerullo,
Hartford Courant,
25 Apr. 2026
Cite this Entry
“Insidious.” Merriam-Webster.com Thesaurus, Merriam-Webster, https://www.merriam-webster.com/thesaurus/insidious. Accessed 1 May. 2026.
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